For one thing you need to consider the abrasive action of the floor moving over a long period of time. How will that affect the barrier?

You also must consider going outside of the recommended installation parameters. If you stay within them you at least have a shot at getting someone from the manufacturer to listen to you.

And besides those issues, you are trying to determine the effect concrete rH has on a wood floor, and technically it has NONE. The wood will be impacted by the moisture coming from the concrete as it tries to equilibrate with the surrounding conditions. If we take the MVER out of the equation (by using a very low perm rate poly sheet or an epoxy mitigation system) you really don't care what the rH is.

The relative humidity of a concrete slab is a fantastic indicator if the slab's potential for emitting moisture. There are many factors that determine the rate of emission and more factors that determine if that rate is acceptable for the flooring.

I can tell you I can treat that slab and guarantee you will have no emission problems up to 100% rH and / or 25lbs MVER. What I cannot tell you is if you can put down brand X poly and make it work on a slab at 90%rH with a high vapor drive because the slab is porous, the ambient is low rH and the temps are high. If I added in multiple unsealed overlap joints and wear from movement along with no underslab vapor retarder, you would be looking at a failure waiting to happen.

So to recap: Relative humidity is the reservoir of moisture waiting to cause trouble. The poly sheet is the barrier keeping it from your floor. Contributing players are the ambient conditions, the quality of the barrier install and the surface porosity of the concrete. Then we have the barrier wear over time factor to consider.

Besides, Koster and Ardex cost less than $1.50/sf. They soak into the concrete and wear like iron. How huge is this job that you can't consider a warranted system for so little cost?

Oh, just so you know this: Koster VAP1-2000FS has a perm rating of 0.1 lbs/1000/24 or 0.05 grains/1hr/ft2 .

If this is a floating floor and not adhered then first off you should test the slab. If it close to 85% I would do a coat of Bostik MVP4, then a six mil plastic. Then float to your hearts content. Never had a failure with that even below grade using solid hardwood with a floating ply subfloor.

Heck even the new Bostik Single App says it will warranty a slab below 85% with no barrier under the slab.

Hey,
When preparing to install solid hardwood flooring, be sure that you do not install over an area with radiant heat. 15 lbs. asphalt felt must be laid to install, and a moisture barrier of 6 mil polyethylene film may also be necessary. This protects against moisture rising up from below and also helps prevent squeaking. Install the vapor barrier parallel to the direction of the flooring.

I'm not a flooring installer so forgive me if this is an elementary question, but how do you install solid wood flooring over plastic over concrete? Do you have to install a plywood substrate to nail the floor to?

I've done solid over concrete in different ways. One was below grade, Bostik MVP4, six mil plastic, floating plywood floor, 15lb felt then nailed down 3/4 Beech, a very unstable solid. The home had a very magnificent HVAC system so you could set it to high and low temps with humidification. The pad was cut into the side of the mountain, multiple levels below grade. Water was seeping out of the granite rock at the bottom of the house. No problemo so far and that was 2 years ago. knock on wood.

Other solids can easily be glued to concrete with high grade adhesives. Lots of shorts have always been glued direct to concrete but no below grade.

Larger/longer wood needs to be milled perfectly in order to get a tight fit when gluing direct. You can cull out quite a few square feet of wood if they are not milled perfectly and high grade mature hardwoods. There are several quality hardwoods that allow you to glue direct but your going to be paying up around $20 a sf, especially for the really wide stuff milled out of old growth lumber.

Theres a guy up in Minnesota that swears he can do almost any solid over radiant heat and he is very well established hardwood guy. I'm just glad we don't go there down here in the Old Pueblo.

Down here we mostly go 12 or 13mm overlay on 12mm ply, so it can be secret nailed without the cleats hitting the slab. Most guys use thin sheet plastic and no vapour barrier. But as I've always said, we under-do things here and slowly learning from the failures. I'll start telling my customers about how you do it and watch the colour drain from their faces.

Direct stick is coming back into fashion but not many will do direct stick solid as it's hard to settle, especially the long boards.

The problem with socialism is that you soon run out of other people's money.
- Margaret Thatcher

Thanks, the look was good but I had to go through tons of wood to get that floor right because the wood had dried down so much it made the milling off. So each row was carefully end matched.

And then there was the stairs. Ouy. All pies on the big one, and concrete on the other that I had to lay plywood on. Plus protect the ply from vapor emissions on the stairs.

Thats the kinda jobs I like, ones that test your skills and competence. Not that it was anything exorbitant like borders and insets. Just fun. I think the price for that big room ran about $14 a sqft U.S. Labor only. The slab was horribly rough and needed a thorough grinding before the MVP4 went down. Plus I used the Wagner Rapid Rh, and Cacl. Double checked them before and after the application of the MVP4. Then checked after it was covered with 6mil plastic. Ran CaCl tests after the application of 6 mil plastic to see the differences in MVER on CaCl with the slab totally covered, duct taped around the domes.

I'm gonna shoot for the Carlisle Master Certification. These people know wood and are not afraid to have you glue solid direct to concrete. Maybe I can get some decent jobs out of it.

I might need to advise them about proper moisutre testing though. A simple layer of 6 mil is not for me. Plus I don't approve of shooting my vapor retarder full of holes when fastening plywood to concrete.